The present invention relates to improved core-shell polymers of the type disclosed in concurrently filed U.S. patent application, Ser. No. 115,824, now abandoned entitled "CORE-SHELL POLYMERS AND PROCESS"; and more particularly, the invention relates to core-shell polymers of this type which have superior against ultraviolet light.
In the above-identified U.S. application, there is disclosed a new organic high polymer composition and a process for preparing it. The compositions disclosed therein comprise discrete particles having cores of one polymer completely covered with shells of a different polymer. The shell will preferably be a methyl methacrylate, ethyl methacrylate, butyl methacrylate, or mixed alkyl methacrylate polymer. The core will preferably be styrene-butadiene (SBR) copolymer. By properly selecting the shell monomer, core particle size and reaction conditions to provide a core-shell number of greater than 10, it is possible to concentrate the acrylate polymer near the shell and leave the center of the core free from the shell polymer. These polymers have been suggested for use as screen print binders and those having styrene-butadiene copolymer cores and polyalkyl acrylate/methacrylate shells have been specifically found usable for this purpose because they exhibit better cyclic aging and redispersion properties than the styrene-butadiene copolymer by itself and are less expensive than the polyalkyl acrylate/methacrylate alone.
Screen printing is a process for applying patterns to textile materials. According to this procedure, a rotary drum having a screen printing mask similar to a silk screening mask, is brought into contact with a fabric and has a composition comprising an aqueous thickened dispersion of pigment and binder applied to the outside of the mask. The composition is applied in a predetermined pattern by virtue of the printing of the mask, and the patterned fabric is then heated to fix the design on the fabric.
Prior to the invention described in the above-identified U.S. patent application, print paste binders containing SBR latex were employed to a limited extent. It was found that these binders exhibited a good hand but had poor water redispersibility, thermal stability and stability to ultraviolet light. This led to processing difficulties, poor crock fastness, and poor cyclic aging. Polymerization of acrylic monomers on the surface of the SBR particles according to the disclosure of the above-identified patent application was found to improve water redispersibility and cyclic aging but was found to decrease the hand of the final fabric products. Moreover, the increase in heat and ultraviolet light stability, expressed in terms of cyclic aging, was not always sufficient for all applications.
According to the disclosure of copending U.S. patent application Ser. No. 016,133 filed Mar. 1, 1979, now abandoned, there are disclosed SBR's with improved heat stability and a process for preparing them. It is disclosed therein that incorporating a suitable antioxidant into the monomer mixture prior to polymerization yields an SBR polymer with improved heat stability.
It is also known that some improvement in stability to ultraviolet light can be obtained for polyalkyl acrylate/methacrylate polymers by incorporating a copolymerizable ultraviolet light stabilizer in the monomer mixture prior to polymerization. The inclusion of these stabilizers is not, however, known in core-shell polymers having SBR cores, and there is no indication in the prior art that improved ultraviolet light stability could be obtained when used in particular core-shell configurations, or that other core-shell polymer properties could be improved through their use.
While screen print binder compositions comprising core-shell polymers of the type described in above-identified application, Ser. No. 115,824, now abandoned have a desirable balance of cyclic aging, redispersibility and cost as compared to previously known binders, experience has shown that most of the compositions disclosed therein cause the textile material to which they are applied to become unduly stiff.
There remains a need for improved SBR core-polyalkyl acrylate/methacrylate shell polymers, especially for use as screen print binders which leave the printed fabric soft in the hand, as well as provide good cyclic aging.